The invention concerns a method of generating ozone in accordance with the preamble to claim 1 and a device for generating ozone in accordance with the preamble to claim 2.
Ozone is a powerful oxidizing agent for organic substances and for inorganic compounds which contain elements with several oxidation degrees. Of the multiple fields of application for ozone, its use for water conditioning is to be mentioned among other things.
Technically, ozone can be generated by silent discharge in an oxygen-containing gas. Silent discharge is, in contrast to spark discharge, to be understood as a stable plasma discharge or coronal discharge. Molecular oxygen is dissociated into atomic oxygen. The reactive oxygen atoms subsequently attach themselves in an exothermic reaction to molecular oxygen and form tri-atomic oxygen molecules, accordingly ozone. The ozone yield is dependent among other things on the electric field strength and operating temperature. Moreover, a dependence on gas composition has been observed. The dependence on operating temperature rests on the fact that ozone at higher temperatures decomposes more rapidly again into molecular oxygen and, due to the accordingly dictated displacement of the equilibrium between the originating and disintegrating ozone, the available ozone concentration is less.
Higher field strengths, which likewise lead to an increased ozone yield, can among other things be achieved through a diminution of the gap and through the selection of dielectrics with higher relative dielectricity constants. For dielectrics with high relative dielectricity constants, doped glasses or ceramic materials enter the question. To be sure, dielectrics of ceramic materials have the drawback that they are inhomogeneous and can practically have a lower puncture strength than homogeneous materials. Furthermore, high-grade ceramic materials in the form of formed bodies with high dimensional stability are extremely expensive. Thinner dielectrics, furthermore, increase the risk of dielectric puncture.
Limits have been established for diminution of the gap due to inescapable manufacturing tolerances along with bending and buckling due to mechanical stresses and heat expansion in operation.
Since a field-strength increase by diminution of the gap width and by using dielectrics with high dielectricity constants leads to a considerable rise in the manufacturing cost, economic limits have here been established.
With DE 38 19 304 C2 as a point of departure, basic to the invention is the task of describing a method and a device with which the ozone yield can be further increased at a comparable employment of energy.
This task is solved in a method according to the present invention and in a device according to the present invention.
The success of the invention rests on several physical influences.
Due to the electrically and thermally conductive arrangement it is achieved that, during forced cooling of the electrodes, the heat that occurs due to the discharge as well as due to the exothermic reaction of the atomic with the molecular oxygen will be better conveyed in the gap between the electrode and the dielectric as well as out of the dielectric since, on the one hand, a direct heat-conducting connection exists between the electrode and the dielectric and, on the other, the area of heat transmission to the throughflowing gas is substantially increased while the heat transmission path to all the points inside the gap is decreased. Since ozone has the tendency to disintegrate again with increasing temperature so that a temperature-dependent equilibrium adjusts itself between the ozone content and the oxygen content, effective cooling can diminish the disintegration of ozone and accordingly improve the yield.
In contrast to a normal gap, which the oxygen-containing gas and the generated ozone flows through in an almost laminar flow, there is forced, due to the electrically and thermally conductive gas permeable arrangement, a turbulent flow in the gap with the consequence that the gas molecules also arrive over and over against the surface of the directly coolable electrodes and therefore give off heat better.
Another improvement in the ozone yield derives from the electrode surface, enlarged by means of the electrically and thermally conductive gas-permeable arrangement, and the effective diminution of the gap. Due to the electrically and thermally conductive gas permeable arrangement it is no longer only the surface of the electrodes itself that is available for the discharge, but also the surface of the arrangement and its ionized environment.
Also achieved due to the arrangement, which fills the real gap, is a smaller, locally different electrically effective gap. The local fields accordingly vary considerably and frequently along the stretch of path of the throughflowing gas, and a good yield of ozone-containing gas is achieved.
Next to the purely surface enlargement and gap diminution, however, there is in addition a physical effect that is designated a hollow-cathode effect and is usually undesired for the generation of large-area homogeneous plasmas.
As to a hollow-cathode effect, it is a matter of an intensive plasma discharge inside a hollow space that is surrounded by the same potential. In the present invention, however, this effect is advantageously utilized in that the hollow spaces are in connection one with another and, thus, the more powerfully generated ozone inside the hollow spaces can also be transported farther. The hollow-cathode effect leads also to a greater increase in ozone formation than could be expected just from the enlarged surface of the electrically and thermally conductive gas permeable arrangement and the diminishment of the partial gap distance.
The electrically and thermally conductive gas-permeable arrangement leads also to better gas mixing. Thereby the probability is also increased that the oxygen molecules become dissociated by the electric field and the result oxygen atoms subsequently enter into the bonding with molecular oxygen into ozone. The ozone yield is also increased by this effect.
Instead of just one gap there can also be provided two gaps that are constituted between a first electrode and a second electrode and separated by a common dielectric. Both gaps are filled by an electrically and thermally conductive gas-permeable arrangement. This is an especially economical solution for increasing the ozone yield in that doubling of the gap is achieved with doubling the electrodes and the dielectric.
A first embodiment of the invention provides that the first and second electrode and the dielectric are plate-shaped. The electrically and thermally conductive gas-permeable arrangement can in this event also advantageously be a spreader in order to compensate for insufficient inherent rigidity on the part of the electrodes or dielectric.
In one alternative embodiment of the invention the first and second electrode and the dielectric are constructed cylindrically symmetrically to each other. In this event the dielectric is disposed in a space enclosed by the first electrode and the second electrode is disposed in a space enclosed by the dielectric. Here as well the electrically and thermally conductive gas-permeable arrangement can be advantageously employed as a spreader and for centering.
In the cylindrically symmetrical embodiment of the device in accordance with the invention, execution of the second electrode with a polygonal surface is of advantage. Assembly of the electrically and thermally conductive gas-permeable arrangement will then be facilitated in that during oversweep of the second electrode the frictional forces will be smaller than with a circular surface. The arrangement can on the other hand yield toward the flat regions between the edges if radial forces become active or deviations occur in the straight-line orientation or diameter of the dielectric or electrode.
The second electrode can be constructed solid or hollow. With the solid construction, a larger cross-section is available along the axial direction for the heat dissipation, whereas the hollow execution allows cooling by means of a coolant.
The first electrode is preferably disposed in a coolant. The range of temperatures necessary for a high ozone yield can, due to forced cooling, be maintained even during high loss output in the gap. This measure is especially effective with a gap lying outside the dielectric.
The heat dissipation from the second electrode can be improved when, in the event of its hollow construction, the hollow region has a coolant flowing through it. This measure is especially effective with a gap lying inside the dielectric. With two gaps, optimal results are obtained when the first electrode is disposed in a coolant and the second electrode has a coolant flowing through it.
Instead of two rigid electrodes, the inside of the cylindrical dielectric can be completely filled with the electrically and thermally conductive gas-permeable arrangement and the arrangement can simultaneously constitute the second electrode. In this way an especially adaptable combination of electrode and electrically and thermally conductive gas-permeable arrangement can be achieved.
In the event of a device with two gaps, they can be aligned parallel to or in series with the flow of gas. With parallel alignment there will be a larger flow cross-section whereas with serial alignment a longer reaction way will be made available.
The electrically and thermally conductive gas-permeable arrangement can comprise chips, granulate, wire, or porous bodies, whereby the wire execution in turn can be a tangle, plait, woven, non-woven, or knit.
With the employment of granulated, it can be a matter of porous granulate, due to which the surface of the inner hollow spaces can be exploited for ozone generation.
Furthermore, it is also possible to employ several porous bodies adapted to the gap or one porous body instead of granulate. These bodies can be inserted separately into the gap, connected to the electrode pointing toward the gap, or even integrated into the surface of the electrode.
It has turned out that a wire knit is especially advantageous because it can be manufactured reproducibly and therefore the properties of the ozone-generator element are also precisely reproducible.
With the cylindrically symmetrical execution of the device the knit can be manufactured in a stocking-like shape and, during assembly, simply drawn over the two electrodes, meaning the inner electrode and/or over the dielectric. The knit will simultaneously assume the centering of the inner electrode with respect to the dielectric and of the dielectric with respect to the first electrode, meaning the outer electrode, so that the use of the otherwise common centering element can be done without.
One practical embodiment of the device in accordance with the invention provides that the electrically and thermally conductive gas-permeable arrangement fills about 1 to 50% and preferably 4 to 20% of the gap volume. When wire is used, it will for practical purposes exhibit a cross-sectional area smaller than 0.2 mm2 and preferably smaller than 0.03 mm2.
By adapting the mesh number, mesh size, and wire thickness as well as the knit density, the number of knit wires and the number of knit layers, it is possible to establish the impedance of the arrangement, to adapt the size of the discharge spaces for maximal ozone production, to regulate the turbulence and intermixture of the gas, and to optimize cooling and heat dissipation. With two parallel gaps they can thus also be adapted for optimal ozone yield.
It is practical that the electrically and thermally conductive gas-permeable arrangement consists of oxygen- and ozone-resistant material. This measure provides for uniform ozone-generating properties and maintenance freedom.